The present invention relates to a control system for an induction motor, and more particularly to a control system for controlling a PWM inverter so that the PWM inverter drives an induction motor at a variable voltage and a variable frequency (namely, a variable motor speed) while keeping constant a ratio of the applied voltage of the motor to the input frequency thereof.
In a case where an induction motor is driven by an inverter, the constant V/f ratio control method is widely used, in which the induction motor is driven at a variable speed while keeping constant the ratio of an A.C. voltage V applied to the induction motor to the frequency f of the A.C. voltage. In the constant V/f ratio control method, however, when the load of the motor is decreased, a superfluous magnetizing current flows through the motor, and thus it is impossible to drive the motor efficiently. In order to solve this problem, various methods have been proposed. For example, a system for controlling the ratio of the output frequency of an inverter to the output voltage thereof is disclosed in Japanese Patent Application No. (JP-A-57-183,297) filed on May 6, 1981 by Toshiba Corporation. In almost all of the above methods, the input current of the inverter (namely, a D.C. current) or the primary current of the motor is detected, and the output voltage of the inverter is controlled so as to be proportional to the value of detected current.
In conventional control systems, the motor current (namely, primary current) does not always correspond to the load of the motor, and hence the motor cannot be driven efficiently. That is, even in, no load condition, a fairly high voltage will be applied to the motor, depending on a certain value of the voltage and frequency set for the motor. In this case, the magnetizing current increases, and the motor current (namely, the primary current) also becomes large. When the voltage applied to the motor is increased in the above state so as to be proportional to the current value, the motor current is more increased since the exciting coil of the motor is put in an over excitation state so as to be in a saturate condition. Accordingly, the efficiency of the motor is lowered, and an over current flows through the motor. Then, a breaker for connecting the inverter to an A.C. power source is made open. Thus, the inverter is put in such a state as PWM signals are stopped to apply to the inverter, and then cannot drive the motor.
Further, according to the conventional control systems, the magnetic flux is controlled by varying the input voltage, and hence a desired torque is generated later than an input voltage by an amount corresponding to a secondary time constant. Accordingly, when the load of the motor increases abruptly, the motor speed decreases greatly, and the slip is increased. Thus, the primary current is increased, and this leads the PWM inverter to the overcurrent trip state. The above-mentioned problems are caused by the fact that the detected motor current does not always correspond to the load of the motor.